Re: safe to defrag XFS on live system?

[Mark Nelson <mark.nelson@inktank.com>]

On 09/14/2012 01:56 PM, Nick Couchman wrote:
>>
>> Hi Guys,
>>
>> There was a change 2.6.38 to the way that speculative preallocation
>> works that basically lets small writes behave like allocsize is not set,
>> and large writes behave like a large one is set:
>>
>> http://permalink.gmane.org/gmane.comp.file-systems.xfs.general/38403
>>
>> Having said that, I had my test gear all ready to go so I decided to
>> give it a try:
>>
>> Setup:
>>
>> - 1 node
>> - 6 OSDs with 7200rpm data disks.
>> - Journals on 2 Intel 520 SSDs (3 per SSD)
>> - LSI SAS2008 Controller (9211-8i)
>> - Network: Localhost
>> - Ceph 0.50
>> - Ubuntu 12.04
>> - Kernel 3.4
>> - XFS mkfs options: -f -i size=2048
>> - Common XFS mount options: -o noatime
>> - No replication
>> - 8 concurrent rados bench instances.
>> - 32 concurrent 4MB ops per instance (256 concurrent ops total)
>>
>> Without allocsize=4M:
>>
>> 781.454MB/s
>>
>> With allocsize=4M:
>>
>> 453.335MB/s
>>
>> I'm guessing that it's perhaps slower as we've told XFS to optimize for
>> large files, but the metadata in /meta is very small, and we were
>> already getting benefits from the new speculative preallocation patches
>> that were introduced in 2.6.38 to combat fragmentation of the 4MB objects.
>>
>> Mark
>
> Interesting, thanks for the results, Mark.  So, I guess don't tune unless you have a very good reason to do so?  Or, if you're really going to try to squeeze all the performance possible, put your metadata on a separate FS with a different alloc size (or no alloc size specified) so that metadata access isn't adversely impacted by trying to tune data access?
>
> -Nick

Well, the XFS guys certainly suggest default tuning in most cases... :)

http://xfs.org/index.php/XFS_FAQ#Q:_I_want_to_tune_my_XFS_filesystems_for_.3Csomething.3E

I think there is value in investigating things when you suspect a 
problem though!

We've tried putting the meta directory on alternate partitions (note: 
this isn't a good idea with btrfs). It hasn't really done much in some 
of the tests we've done, but we weren't looking at testing this specific 
scenario.

I think the bigger question is, what problem are you trying to solve? 
Are you noticing lots of fragmentation?  Slow performance with 4MB 
writes?  slow performance with small IO?

>
>
>
> --------
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Mark